Overview Locating Genes Along Chromosomes Mendels hereditary factors

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Overview: Locating Genes Along Chromosomes § Mendel’s “hereditary factors” were genes § Today we

Overview: Locating Genes Along Chromosomes § Mendel’s “hereditary factors” were genes § Today we know that genes are located on chromosomes § The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene © 2016 Pearson Education, Inc.

§ Mitosis and meiosis were first described in the late 1800 s § The

§ Mitosis and meiosis were first described in the late 1800 s § The chromosome theory of inheritance states § Mendelian genes have specific loci (positions) on chromosomes § Chromosomes undergo segregation and independent assortment § The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment © 2016 Pearson Education, Inc.

Figure 12. 2 P Generation Yellow round seeds (YYRR) Y r R R Y

Figure 12. 2 P Generation Yellow round seeds (YYRR) Y r R R Y Green wrinkled seeds (yyrr) y r y Meiosis Fertilization R Y Gametes y r All F 1 plants produce yellow round seeds(Yy. Rr). F 1 Generation R R y r Y Y Meiosis LAW OF SEGREGATION The two alleles for each gene separate. R r Y y r R Y y Metaphase I LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently. r R Y y Anaphase I R r Y y R R 1 4 YR F 2 Generation Fertilization recombines the R and r alleles at random. © 2016 Pearson Education, Inc. y Y Y Metaphase II 4 Y y Y r r 1 R Y y r r r 1 yr 4 Yr An F 1 cross-fertilization 9 : 3 : 1 y y R R 1 4 y. R Fertilization results in the 9: 3: 3: 1 phenotypic ratio in the F 2 generation.

Concept 12. 1: Morgan showed that Mendelian inheritance has its physical basis in the

Concept 12. 1: Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: scientific inquiry § The first solid evidence associating a specific gene with a specific chromosome came from the work of Thomas Hunt Morgan in the early 1900 s © 2016 Pearson Education, Inc.

Morgan’s Choice of Experimental Organism § Morgan selected a species of fruit fly, Drosophila

Morgan’s Choice of Experimental Organism § Morgan selected a species of fruit fly, Drosophila melanogaster, as his research organism § Several characteristics make fruit flies a convenient organism for genetic studies § They produce many offspring § A generation can be bred every two weeks § They have only four pairs of chromosomes © 2016 Pearson Education, Inc.

§ Morgan noted wild-type, or normal, phenotypes that were common in the fly populations

§ Morgan noted wild-type, or normal, phenotypes that were common in the fly populations § Traits alternative to the wild type are called mutant phenotypes § The first mutant phenotype he discovered was a fly with white eyes instead of the wild type red © 2016 Pearson Education, Inc.

Figure 12. 3 © 2016 Pearson Education, Inc.

Figure 12. 3 © 2016 Pearson Education, Inc.

Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair § In

Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair § In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) § The F 1 generation all had red eyes § The F 2 generation showed the classical 3: 1 red: white ratio, but only males had white eyes § Morgan concluded that the eye color was related to the sex of the fly © 2016 Pearson Education, Inc.

§ Morgan determined that the white-eyed mutant allele must be located on the X

§ Morgan determined that the white-eyed mutant allele must be located on the X chromosome § Morgan’s finding supported the chromosome theory of inheritance © 2016 Pearson Education, Inc.

Figure 12. 4 -2 Conclusion P Generation X X w X Y w w

Figure 12. 4 -2 Conclusion P Generation X X w X Y w w F 1 Generation Sperm Eggs w w w Eggs F 2 Generation w w Sperm w w w w © 2016 Pearson Education, Inc. w

Concept 12. 2: Sex-linked genes exhibit unique patterns of inheritance § The behavior of

Concept 12. 2: Sex-linked genes exhibit unique patterns of inheritance § The behavior of the members of the pair of sex chromosomes can be correlated with the behavior of the two alleles of the eye-color gene white © 2016 Pearson Education, Inc.

The Chromosomal Basis of Sex § Humans and other mammals have two types of

The Chromosomal Basis of Sex § Humans and other mammals have two types of sex chromosomes: a larger X chromosome and a smaller Y chromosome § Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome § The SRY gene on the Y chromosome is required for the developments of testes © 2016 Pearson Education, Inc.

Figure 12. 5 X Y © 2016 Pearson Education, Inc.

Figure 12. 5 X Y © 2016 Pearson Education, Inc.

Figure 12. 6 44 XY Parents 22 X Egg 22 or X Y Sperm

Figure 12. 6 44 XY Parents 22 X Egg 22 or X Y Sperm 44 XX or 44 XY Zygotes (offspring) © 2016 Pearson Education, Inc.

§ A gene that is located on either sex chromosome is called a sex-linked

§ A gene that is located on either sex chromosome is called a sex-linked gene § Genes on the Y chromosome are called Y-linked genes; there are few of these § Genes on the X chromosome are called X-linked genes © 2016 Pearson Education, Inc.

Inheritance of X-Linked Genes § Most Y-linked genes help determine sex § The X

Inheritance of X-Linked Genes § Most Y-linked genes help determine sex § The X chromosomes have genes for many characters unrelated to sex © 2016 Pearson Education, Inc.

§ X-linked genes follow specific patterns of inheritance § For a recessive X-linked trait

§ X-linked genes follow specific patterns of inheritance § For a recessive X-linked trait to be expressed § A female needs two copies of the allele (homozygous) § A male needs only one copy of the allele (hemizygous) § X-linked recessive disorders are much more common in males than in females © 2016 Pearson Education, Inc.

Figure 12. 7 XN XN Xn Eggs XN XN Y Sperm XN Xn XN

Figure 12. 7 XN XN Xn Eggs XN XN Y Sperm XN Xn XN Y XN XN Xn X N Y (a) XN Xn X n. Y XN Xn XN Y Y Sperm Xn X n. Y Y Eggs XN XNY Eggs XN XN Xn XN Y X N X n. Y Xn X n. Y (b) Xn © 2016 Pearson Education, Inc. (c) Sperm

§ Some disorders caused by recessive alleles on the X chromosome in humans: §

§ Some disorders caused by recessive alleles on the X chromosome in humans: § Color blindness (mostly X-linked) § Duchenne muscular dystrophy § Hemophilia © 2016 Pearson Education, Inc.

X Inactivation in Female Mammals § In mammalian females, one of the two X

X Inactivation in Female Mammals § In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development § The inactive X condenses into a Barr body § If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character © 2016 Pearson Education, Inc.

Figure 12. 8 X chromosomes Early embryo: Two cell populations in adult cat: Active

Figure 12. 8 X chromosomes Early embryo: Two cell populations in adult cat: Active X Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Black fur © 2016 Pearson Education, Inc. Allele for orange fur Orange fur

Concept 12. 3: Linked genes tend to be inherited together because they are located

Concept 12. 3: Linked genes tend to be inherited together because they are located near each other on the same chromosome § Each chromosome has hundreds or thousands of genes (except the Y chromosome) § Genes located on the same chromosome that tend to be inherited together are called linked genes © 2016 Pearson Education, Inc.

How Linkage Affects Inheritance § Morgan did experiments with fruit flies that show linkage

How Linkage Affects Inheritance § Morgan did experiments with fruit flies that show linkage affects inheritance of two characters § Morgan crossed flies that differed in traits of body color and wing size © 2016 Pearson Education, Inc.

§ Morgan found that body color and wing size are usually inherited together in

§ Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) § He reasoned that since these genes did not assort independently, they were on the same chromosome © 2016 Pearson Education, Inc.

Figure 12. UN 01 F 1 dihybrid female and homozygous recessive male in testcross

Figure 12. UN 01 F 1 dihybrid female and homozygous recessive male in testcross b vg b vg Most offspring or b vg © 2016 Pearson Education, Inc. b vg

§ However, nonparental phenotypes were also produced § Understanding this result involves exploring genetic

§ However, nonparental phenotypes were also produced § Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent © 2016 Pearson Education, Inc.

Figure 12. 9 Experiment P Generation (homozygous) Wild type (gray body, normal wings) Double

Figure 12. 9 Experiment P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) b b vg vg F 1 dihybrid testcross Homozygous recessive (black body, vestigial wings) Wild-type F 1 dihybrid (gray body, normal wings) b b vg vg Testcross offspring Eggs b vg Black Wild type (gray normal) vestigial b vg Gray vestigial Black normal b vg Sperm b b vg vg PREDICTED RATIOS Genes on different chromosomes: 1 Genes on same chromosome: 1 : 0 965 : 944 : 206 : 185 Results © 2016 Pearson Education, Inc.

Genetic Recombination and Linkage § The genetic findings of Mendel and Morgan relate to

Genetic Recombination and Linkage § The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination © 2016 Pearson Education, Inc.

Recombination of Unlinked Genes: Independent Assortment of Chromosomes § Mendel observed that combinations of

Recombination of Unlinked Genes: Independent Assortment of Chromosomes § Mendel observed that combinations of traits in some offspring differ from either parent § Offspring with a phenotype matching one of the parental phenotypes are called parental types § Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants § A 50% frequency of recombination is observed for any two genes on different chromosomes © 2016 Pearson Education, Inc.

Figure 12. UN 02 Gametes from yellow round dihybrid parent (Yy. Rr ) Gametes

Figure 12. UN 02 Gametes from yellow round dihybrid parent (Yy. Rr ) Gametes from green wrinkled homozygous recessive parent (yyrr) YR yr Yr y. R Yy. Rr yyrr Yyrr yy. Rr yr Parentaltype offspring © 2016 Pearson Education, Inc. Recombinant offspring

Recombination of Linked Genes: Crossing Over § Morgan discovered that even when two genes

Recombination of Linked Genes: Crossing Over § Morgan discovered that even when two genes were on the same chromosome, some recombinant phenotypes were observed § He proposed that some process must occasionally break the physical connection between genes on the same chromosome § That mechanism was the crossing over between homologous chromosomes © 2016 Pearson Education, Inc.

Animation: Crossing Over © 2016 Pearson Education, Inc.

Animation: Crossing Over © 2016 Pearson Education, Inc.

Figure 12. 10 P generation (homozygous) Wild type (gray body, normal wings) b vg

Figure 12. 10 P generation (homozygous) Wild type (gray body, normal wings) b vg Double mutant (black body, vestigial wings) b vg F 1 dihybrid testcross b Wild-type F 1 dihybrid (gray body, normal wings) b vg Homozygous recessive (black body, vestigial wings) b vg Replication of chromosomes vg b vg Replication of chromosomes b vg b vg Meiosis I b vg Meiosis I and II b vg Recombinant chromosomes Meiosis II Eggs Testcross offspring b vg 965 Wild type 944 Black vestigial b vg 185 Black normal b vg b vg (gray normal) Parental-type offspring 206 Gray vestigial Recombinant offspring 391 recombinants © 2016 Pearson Education, Inc. b vg Recombination × 100 17% frequency 2, 300 total offspring b vg Sperm

New Combinations of Alleles: Variation for Natural Selection § Recombinant chromosomes bring alleles together

New Combinations of Alleles: Variation for Natural Selection § Recombinant chromosomes bring alleles together in new combinations in gametes § Random fertilization increases even further the number of variant combinations that can be produced § This abundance of genetic variation is the raw material upon which natural selection works © 2016 Pearson Education, Inc.

Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry § Alfred Sturtevant, one

Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry § Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome § Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency © 2016 Pearson Education, Inc.

§ A linkage map is a genetic map of a chromosome based on recombination

§ A linkage map is a genetic map of a chromosome based on recombination frequencies § Distances between genes can be expressed as map units; one map unit represents a 1% recombination frequency © 2016 Pearson Education, Inc.

Figure 12. 11 Results Recombination frequencies 9% Chromosome 17% b © 2016 Pearson Education,

Figure 12. 11 Results Recombination frequencies 9% Chromosome 17% b © 2016 Pearson Education, Inc. 9. 5% cn vg

§ Genes that are far apart on the same chromosome can have a recombination

§ Genes that are far apart on the same chromosome can have a recombination frequency near 50% § Such genes are physically connected, but genetically unlinked © 2016 Pearson Education, Inc.

§ Sturtevant used recombination frequencies to make linkage maps of fruit fly genes §

§ Sturtevant used recombination frequencies to make linkage maps of fruit fly genes § Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes § Cytogenetic maps indicate the positions of genes with respect to chromosomal features © 2016 Pearson Education, Inc.

Figure 12. 12 Mutant phenotypes Short aristae 0 Maroon eyes 16. 5 Red Long

Figure 12. 12 Mutant phenotypes Short aristae 0 Maroon eyes 16. 5 Red Long eyes aristae (appendages on head) © 2016 Pearson Education, Inc. Black Cinnabar eyes body Gray body Vestigial wings Down- Brown curved eyes wings 48. 5 57. 5 67. 0 75. 5 104. 5 Red eyes Normal wings Wild-type phenotypes Normal wings Red eyes

Concept 12. 4: Alterations of chromosome number or structure cause some genetic disorders §

Concept 12. 4: Alterations of chromosome number or structure cause some genetic disorders § Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders § Plants tolerate such genetic changes better than animals do © 2016 Pearson Education, Inc.

Abnormal Chromosome Number § In nondisjunction, pairs of homologous chromosomes do not separate normally

Abnormal Chromosome Number § In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis § As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy © 2016 Pearson Education, Inc.

Figure 12. 13 -s 3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n 1

Figure 12. 13 -s 3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n 1 n 1 n 1 n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I © 2016 Pearson Education, Inc. (b) Nondisjunction of sister chromatids in meiosis II n

§ Aneuploidy results from fertilization involving gametes in which nondisjunction occurred § Offspring with

§ Aneuploidy results from fertilization involving gametes in which nondisjunction occurred § Offspring with this condition have an abnormal number of a particular chromosome © 2016 Pearson Education, Inc.

§ A monosomic zygote has only one copy of a particular chromosome § A

§ A monosomic zygote has only one copy of a particular chromosome § A trisomic zygote has three copies of a particular chromosome © 2016 Pearson Education, Inc.

§ Polyploidy is a condition in which an organism has more than two complete

§ Polyploidy is a condition in which an organism has more than two complete sets of chromosomes § Triploidy (3 n) is three sets of chromosomes § Tetraploidy (4 n) is four sets of chromosomes § Polyploidy is common in plants, but not animals § Polyploids are more normal in appearance than aneuploids © 2016 Pearson Education, Inc.

Alterations of Chromosome Structure § Breakage of a chromosome can lead to four types

Alterations of Chromosome Structure § Breakage of a chromosome can lead to four types of changes in chromosome structure § Deletion removes a chromosomal segment § Duplication repeats a segment § Inversion reverses orientation of a segment within a chromosome § Translocation moves a segment from one chromosome to another © 2016 Pearson Education, Inc.

Figure 12. 14 (c) Inversion (a) Deletion A B C D E F G

Figure 12. 14 (c) Inversion (a) Deletion A B C D E F G H A B C D E An inversion reverses a segment within a chromosome. A deletion removes a chromosomal segment. A B C E A D C B E F G H (b) Duplication F G H (d) Translocation A B C D E F G H A duplication repeats a segment. A B C D E F G H R A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C D E © 2016 Pearson Education, Inc. M N O P Q F G H A B P Q R

§ A diploid embryo that is homozygous for a large deletion is likely missing

§ A diploid embryo that is homozygous for a large deletion is likely missing a number of essential genes; such a condition is generally lethal § Duplications and translocations also tend to be harmful § In inversions, the balance of genes is normal but phenotype may be influenced if the expression of genes is altered © 2016 Pearson Education, Inc.

Human Disorders Due to Chromosomal Alterations § Alterations of chromosome number and structure associated

Human Disorders Due to Chromosomal Alterations § Alterations of chromosome number and structure associated with some serious disorders § Some types of aneuploidy upset the genetic balance less than others, resulting in individuals surviving to birth and beyond § These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy © 2016 Pearson Education, Inc.

Down Syndrome (Trisomy 21) § Down syndrome is an aneuploid condition that results from

Down Syndrome (Trisomy 21) § Down syndrome is an aneuploid condition that results from three copies of chromosome 21 § It affects about one out of every 830 children born in the United States § The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained © 2016 Pearson Education, Inc.

Figure 12. 15 © 2016 Pearson Education, Inc.

Figure 12. 15 © 2016 Pearson Education, Inc.

Aneuploidy of Sex Chromosomes § Nondisjunction of sex chromosomes produces a variety of aneuploid

Aneuploidy of Sex Chromosomes § Nondisjunction of sex chromosomes produces a variety of aneuploid conditions § Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals § About one in every 1, 000 males is born with an extra Y chromosome (XYY) and does not exhibit any defined syndrome § Females with trisomy X (XXX) have no unusual physical features except being slightly taller than average © 2016 Pearson Education, Inc.

§ Monosomy X, called Turner syndrome, produces X 0 females, who are sterile §

§ Monosomy X, called Turner syndrome, produces X 0 females, who are sterile § It is the only known viable monosomy in humans © 2016 Pearson Education, Inc.

Disorders Caused by Structurally Altered Chromosomes § The syndrome cri du chat (“cry of

Disorders Caused by Structurally Altered Chromosomes § The syndrome cri du chat (“cry of the cat”) results from a specific deletion in chromosome 5 § A child born with this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood § Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes © 2016 Pearson Education, Inc.

Figure 12. 16 Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9

Figure 12. 16 Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) © 2016 Pearson Education, Inc.